Magnetite nanoparticles have great potential for use in medical and other applications, so understanding their properties is crucial. A property still left to be understood is the magnetic ordering of assemblies of nanoparticles at the nanoscale. This paper addresses how the magnetic ordering in magnetite nanoparticle assemblies changes as a function of nanoparticle size and external magnetic field at high temperature. Nanoparticle assemblies were fabricated using organic methods and placed on membranes. These samples were put through x-ray resonant magnetic scattering (XRMS) which produced scattering images that provided information about the magnetic ordering of the particles. Various images were obtained using XRMS for different field values and temperatures. These images were reduced to one-dimensional scattering profiles. By fitting these scattering profiles with a model, we found the percentages of ferromagnetic contribution, antiferromagnetic contribution, and the random contribution. There is a large random contribution as the field value approaches 0 Oe for Sample 9, the sample with the smallest particles, at 300 K. For Sample 3, the sample with the largest particles, at 280 K and at 300 K, there is a slight increase in the antiferromagnetic contribution and large random contribution at low field value. The larger particles are thus demonstrating more antiferromagnetic ordering at low magnetic field values than the smaller particles when placed in high temperature. Therefore, our methods yield information about the magnetic ordering of magnetite nanoparticles and the possibility to control the magnetic ordering through particle size.